The equilibration of the system involving two alternatives, enzymically active complementing structures, type I and II simultaneously formed from two overlapping fragments, Nuclease-(1-126) and Nuclease-T-(50-149) of Staphylococcal nuclease has been studied by determining the ratio of type I to type II complex as a function of incubation time, temperature and the presence or absence of ligands. The ratio of type I to type II complex initially formed was approximatley 0.3 and independent of temperature and the presence or absence of ligands. The ratio of type I to type II complex after the two complexes have reached the equilibrium state through unfolding and folding is 1.1 and 2.4 at 6 and 23 degrees, respectively. It is concluded that the rate of folding is not related to the decrease in energy from the unfolded to the folded state. A precursor of nuclease Foggi is found to contain the extra amino acid sequence of Ser-Glx-Thr-Asp-Asx-Gly-Val-Asx-Arg-Ser-Gly-Ser-Glu-Asp-Pro-Thr-Val-Tyr at the NH2-terminus of nuclease. Nuclease-(1-126) saturated with substrates is shown to have approximately one-thousandth the enzymic activity of nuclease.